The present invention relates in general to electric inverter drives for electrified vehicles, and, more specifically, to improving switching characteristics in a multi-phase inverter by enhancing a common source inductance in a gate loop of a power transistor without modification of a power module incorporating the power transistor.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque to the wheels. A typical electric drive system may include a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC linking capacitor. An inverter is connected between the main buses and a traction motor in order to convert the DC bus power to an AC voltage that is coupled to the windings of the motor to propel the vehicle.
The inverter includes transistor switching devices (such as insulated gate bipolar transistors, IGBTs) connected in a bridge configuration with a plurality of phase legs. Each phase leg is constructed as a half bridge with a high-side transistor connected in series with a low-side transistor between the DC buses. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor. The inverter may pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals applied to the gates of the IGBTs turn them on and off as necessary so that the resulting current matches a desired current.
Semiconductor switching devices such as an IGBT or a MOSFET are driven at a gate terminal by a gate signal provided by a driver circuit. For an IGBT, the gate signal is applied between the gate terminal and an emitter terminal of the device. In the ON state, an output signal is conducted through the device between a collector terminal and the emitter terminal. During the ON state, current within the device flows within a gate loop and within a power loop.
Common source inductance refers to an inductance shared by the main power loop (i.e., the drain-to-source or collector-to-emitter power output of the transistor) and the gate loop (i.e., gate-to-source or gate-to-emitter) in a power switching transistor. The common source inductance carries both the device output current (e.g., drain to source current) and the gate charging/discharging current. Due to the coupling between the input and output of the transistor, current in the output (power loop) portion of the common source inductance modifies the gate voltage in a manner that can be used to reinforce (e.g., speed up) the switching performance. The opportunity to reduce switching time may be desirable since it may have an associated reduction in the energy consumed (i.e., lost) during the switching transition. The magnitude of the gate loop inductance and/or the power loop inductance and the degree of mutual coupling between them can be manipulated (e.g., enhanced) by selecting an appropriate layout and/or including added overlapping coils in PCB traces forming conductive paths to the transistor gates or emitters in order to obtain a desired common source inductance, for example. Various examples of structural modifications within a power module to enhance the common source inductance are shown in U.S. patent application publication 2018/0152113A1, U.S. patent application publication 2018/0123478A1, U.S. Pat. Nos. 9,994,110, and 10,122,294, each of which is incorporated herein by reference in their entirety.
A typical inverter system for an electric drive utilizes one or more power modules containing the power switching devices (e.g., IGBTs) and associated components (such as a reverse diode across each IGBT) arranged according to the phase legs of the bridge configuration. The power modules typically generate a large amount of heat, so they are often attached to a coldplate (e.g., a liquid cooled heatsink) for better thermal performance. A controller and the gate driver circuits for the inverter are typically separated from the power module(s) by situating them on a separate circuit board and/or module from the power modules.
Since the common source inductance is a coupling between the gate loop and the power loop of a switching device, known methods for enhancing the common to source inductance have relied on modifications to the power module since that is where the two loops are in close proximity. However, the power module for use in vehicle electric drive systems must satisfy stringent requirements concerning reliability, efficiency, durability, and cost. Another important consideration is packaging size. When changing the power module design and manufacturing process to add structures or components that increase the common source inductance between the gate loop and power loop, the packaging size for the power module has been increased. Furthermore, customized components increase the parts cost and the manufacturing cost of the power module. Therefore, it would be desirable to increase common source inductance without an increase in the number of components in the power module or making any other unique modifications in the power module.